Current control apparatus

ABSTRACT

An apparatus for a load tap changer includes a first primary winding electrically connected to a first contact, the first contact configured to connect to one of a plurality of taps in a load tap changer; a second contact, the second contact configured to connect to one of the plurality of taps in the load tap changer; a magnetic core; and a control circuit including: a secondary winding configured to magnetically couple to the first primary winding and the magnetic core; and an electrical network electrically connected to the secondary winding, the electrical network being configured to prevent magnetic saturation of the magnetic core during switching of the first or second contact.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/531,440, filed Aug. 5, 2019 and titled CURRENT CONTROL APPARATUS,which claims the benefit of U.S. Provisional Application No. 62/719,974,filed on Aug. 20, 2018 and titled CURRENT CONTROL APPARATUS, both ofwhich are incorporated herein by reference in their entirety.

TECHNICAL FIELD

This disclosure relates to a current control apparatus. The currentcontrol apparatus may be used to control current in a winding of adevice used for voltage regulation.

BACKGROUND

Voltage regulators are used to monitor and control a voltage level in anelectrical power distribution network. A voltage regulator includes amain winding and an electromagnetic circuit that delivers current fromthe main winding to an electric load. The electromagnetic circuitincludes electrical contacts, and the main winding includes a pluralityof taps. The output voltage of the voltage regulator is determined bywhich of the plurality of taps are in contact with the electricalcontacts.

SUMMARY

In one general aspect, an apparatus for a load tap changer includes afirst primary winding electrically connected to a first contact, thefirst contact configured to connect to one of a plurality of taps in aload tap changer; a second contact, the second contact configured toconnect to one of the plurality of taps in the load tap changer; amagnetic core; and a control circuit including: a secondary windingconfigured to magnetically couple to the first primary winding and themagnetic core; and an electrical network electrically connected to thesecondary winding, the electrical network being configured to preventmagnetic saturation of the magnetic core during switching of the firstor second contact.

Implementations may include one or more of the following features. Theelectrical network may prevent magnetic saturation of the magnetic coreby reducing the absolute value of magnetic flux in the magnetic core.The absolute value of magnetic flux in the magnetic core may be reducedby allowing the flow of electrical current in the secondary winding. Theelectrical network may be powered by an alternating current (AC) powersource. The AC power source may include a third winding that iselectrically connected to the first primary winding. The electricalnetwork may prevent magnetic saturation of the magnetic core byincreasing or decreasing electrical current in the secondary winding toincrease or decrease the magnetic flux in the magnetic core. Theelectrical network may include a direct current (DC) bus, and electricalpower to increase or decrease electrical current in the secondarywinding is provided by the direct current (DC) bus. The direct current(DC) bus also may be coupled to an alternating current (AC) power systemto compensate reactive power. The alternating current (AC) power systemmay be a multi-phase system. The electrical network may include a powersource, and the power source may be controllable to increase or decreaseelectrical current in the secondary winding. The power source may bepowered from a voltage transformer. The power source may be powered froma current transformer. The increase or decrease of magnetic flux in themagnetic core may cause a circulating current to flow in a shortcircuit, the short circuit being formed by the first contact, the secondcontact, and the primary winding. The circulating current may be equalin amplitude and opposite in phase to a load current carried by thefirst contact or the second contact. The load tap changer may receivepower from an alternating current (AC) power distribution network thatoperates at a system frequency, and causing the circulating current toflow in the short circuit may result in the net current through thefirst contact or the second contact being equal to zero more frequentlythan the system frequency. Causing the circulating current to flow inthe short circuit may reduce the root-mean-square of the net currentthrough the first contact or the second contact.

In some implementations, the apparatus for the load tap changer alsoincludes: a second primary winding connected to the second contact; asecond magnetic core; and a second secondary winding magneticallycoupled to the second magnetic core and second primary winding. In theseimplementations, the electrical network is also connected to the secondsecondary winding and configured to control the current in the firstsecondary winding and second secondary winding. Further, the electricalnetwork may control the current in the first primary winding and thesecond primary winding by controlling the current in the first secondarywinding and the second secondary winding. The current through the firstcontact may be zero while switching taps. The output voltage to aconnected load may be controlled by an electrical network connected tothe first primary winding and the second primary winding.

In another general aspect, an apparatus for controlling voltage outputof a transformer includes a first current path including a first primarywinding electrically connected to a winding tap; a second current pathincluding a second primary winding electrically connected to a windingtap; and an electrical network magnetically coupled to the first primarywinding and second primary winding, the electrical network beingconfigured to control current in the first and the second primarywindings.

Implementations may include one or more of the following features. Thetransformer may be a multi-phase transformer. The electrical network mayinclude a first switch;

a second switch; and a bypass switch connected between the first switchand the second switch. A direct current (DC) bus may be coupled to thetransformer to compensate reactive power from the alternating current(AC) power system.

Implementations of any of the techniques described herein may include avoltage regulator, a load tap changer, an apparatus, a current controlapparatus, a kit for retrofitting an existing voltage regulator with acurrent control apparatus, a controller for controlling a voltageregulator, or a process. The details of one or more implementations areset forth in the accompanying drawings and the description below. Otherfeatures will be apparent from the description and drawings, and fromthe claims.

DRAWING DESCRIPTION

FIG. 1A is a block diagram of an example of an alternating-current (AC)electrical power system.

FIGS. 1B and 1C are block diagrams of an example of a core.

FIG. 2 is a block diagram of an example of a voltage regulator.

FIGS. 3A-3E are block diagrams of another example of a voltageregulator.

FIG. 4 is a block diagram of another example of a voltage regulator.

FIGS. 5 and 6 are examples of simulated data.

FIG. 7A is a block diagram of another example of a voltage regulator.

FIG. 7B is a block diagram of an example of an electrical network.

FIG. 7C is a block diagram of another example of an electrical network.

FIG. 7D is a block diagram of another example of a voltage regulator.

FIG. 8A is a block diagram of another example of a voltage regulator.

FIG. 8B is a block diagram of an another example of an electricalnetwork.

FIG. 9A is a block diagram of another example of a voltage regulator.

FIG. 9B is a block diagram of an example of a current control apparatus.

FIGS. 10A, 10B, and 10E are block diagrams of other examples of avoltage regulator.

FIGS. 10C and 10D are block diagrams of examples of electrical networks.

FIGS. 11 and 12 are block diagram of other examples of a current controlapparatus.

DETAILED DESCRIPTION

FIG. 1A is a block diagram of an example of an alternating-current (AC)electrical power system 100. The electrical power system 100 includes anelectrical power distribution network 101 that transfers electricityfrom a power source 102 to electrical loads 103 through a distributionpath 104 and an electrical apparatus 110. The electrical apparatus 110is any apparatus that is capable of regulating the voltage to the loads103. For example, the electrical apparatus 110 may be a voltageregulator that includes a load tap changer. The electrical powerdistribution network 101 may be, for example, an electrical grid, anelectrical system, or a multi-phase electrical network that provideselectricity to commercial and/or residential customers. The electricalpower distribution network 101 may have an operating voltage of, forexample, at least 1 kilovolt (kV), 12 kV, up to 34.5 kV, up to 38 kV, or69 kV or higher, and may operate at a system frequency of, for example,50-60 Hertz (Hz). The distribution path 104 may include, for example,one or more transmission lines, electrical cables, and/or any othermechanism for transmitting electricity.

The electrical apparatus 110 includes an electromagnetic circuit 120 anda current control circuit 150, which controls a current in theelectromagnetic circuit 120. The electromagnetic circuit 120 includes awinding 121. The winding 121 is an electrical conductor. For example,the winding 121 may be a cable or wire made of an electricallyconductive material, such as a metal. Referring also to FIG. 1B, thewinding 121 is wrapped in, for example, a coil or helical shape having acentral region 122.

In the example shown in FIG. 1B, the winding 121 is wrapped around amagnetic core 123 that is in the central region 122. The magnetic core123 is made of a ferromagnetic material, such as, for example, iron orsteel. In the example of FIG. 1B, the magnetic core is shown as a rod.However, the magnetic core 123 may have any shape. For example, themagnetic core 123 may be a ring, a square or rectangular shaped annulus,or any other structure that has a ferromagnetic region suitable forattaching a winding. The magnetic core 123 may be a gapped core or anun-gapped core. In implementations in which the core 123 is an un-gappedcore, the core 123 is a contiguous segment of ferromagnetic material. Agapped core includes a gap that is not ferromagnetic material. The gapmay be, for example, air, nylon, or any other material that is notferromagnetic. Thus, in implementations in which the core 123 is agapped core, the core includes at least one segment of a ferromagneticmaterial and at least one segment of a material that is not aferromagnetic material. In implementations in which the core has morethan one segment of ferromagnetic material, the segments are separatedfrom each other with a material that is not a ferromagnetic material.The regions of non-ferromagnetic material between the segments offerromagnetic material are referred to as gaps.

The current control circuit 150 controls the current in theelectromagnetic circuit 120 by controlling an amount of magnetic flux inthe magnetic core 123. Magnetic flux is a measure of the total magneticfield that passes through a surface and is defined as is the surfaceintegral of the normal component of a magnetic field that passes througha surface in units of weber (Wb). The relationship between current,voltage, and magnetic flux in an electromagnetic circuit given variousopen-circuit and closed-circuit conditions is fundamental to theoperation of the current control circuit 150. For example, the magneticfield generated by a current that is carried in a wire is given by:

$\begin{matrix}{{B = \frac{\mu_{0}I}{2{\pi r}}},} & {{Equation}\mspace{14mu} (1)}\end{matrix}$

where B is the magnitude of the magnetic field in Teslas (T), μ₀ is thepermeability of free space, I is the magnitude of the current that iscarried in the wire, and r is the distance from the wire in meters (m).The magnetic field in a material that is not free space (such as thecore 123) is related to B by the permeability of the material. As notedabove, the magnetic flux depends on the magnetic field. Thus, the amountof magnetic flux in the core 123 may be controlled by controlling theamount of current in the winding 121 or by controlling the amount ofcurrent in a winding 151, which is also wrapped around the core 123.

The current control circuit 150 includes the winding 151, which is anelectrical conductor (for example, a metal wire). Referring also to FIG.1C, the winding 151 (shown as a dashed line to visually distinguish thewinding 151 from the winding 121) is also wrapped around the magneticcore 123. The winding 151 and the winding 121 are electrically isolatedfrom each other but are magnetically coupled via the magnetic core 123.In a system in which a first coil or winding and a second coil orwinding share a common magnetic core, a time-varying electrical currentin the first winding generates a time-varying magnetic field in thecommon core, and the generated time-varying magnetic field induces acorresponding time-varying current in the second winding, and viceversa. Thus, a time-varying current in the winding 151 generates acorresponding time-varying current in the winding 121 of theelectromagnetic circuit 120. Using contemporary power electronicprinciples, the current control circuit 150 can act as a source or animpedance connected to winding 151 to cause or prevent the flow ofcurrent in winding 121. For example, the current control circuit 150 maybe used to cause the current in the winding 121 to decrease or drop tozero prior to separating a contact connected to the winding 121 from atap. Moreover, the control circuit 150 may be used to control an amountof magnetic flux in the magnetic core 123 during a switching operationto thereby prevent or minimize the likelihood of saturation. Variousimplementations of the current control circuit 150 are discussed below.Prior to discussing the various implementations of the current controlcircuit 150, an overview of a voltage regulator that includes a load tapchanger is provided.

Referring to FIG. 2, a block diagram of a voltage regulator 210 isshown. In the example of FIG. 2, the dash-dot lines indicate a data link259 over which data, such as, for example, information, commands, ornumerical data, travel. Solid lines between blocks indicate a paththrough which current flows between the source 102 and the load 103. Thevoltage regulator 210 includes a load tap changer and is an example ofan implementation of the electrical apparatus 110 (FIG. 1A). The loadtap changer includes taps 215 and electrical contacts 224. The voltageregulator 210 monitors and controls the voltage level at thedistribution path 104 such that the voltage delivered to the electricalloads 103 (FIG. 1A) is maintained within a desired or acceptable voltagerange despite changes in the electrical load 103 and/or changes in thevoltage supplied by the source 102 (FIG. 1A).

The voltage regulator 210 includes a monitoring module 212, a tapselector 213, a main winding 214, and at least two taps 215 electricallyconnected to the main winding 214. The monitoring module 212 may be anytype of device capable of measuring or determining the voltage on thedistribution path 104. For example, the monitoring module 212 may be avoltage sensor. The tap selector 213 may include, for example, motors,mechanical linkages, and/or electronic circuitry that is capable ofconnecting the load 103 to the source 102 through any of the taps 215.The voltage regulator 210 also includes an electromagnetic circuit 220.Together, the taps 215, the main winding 214, the tap selector 213, andthe electromagnetic circuit 220 form a voltage regulation operationmodule 216 for the voltage regulator 210.

The tap selector 213 is configured to move an electrical contact 224 andplace the electrical contact 224 on a particular one of the taps 215.When one or more of the electrical contacts 224 is connected to one ormore of the taps 215, the electromagnetic circuit 220 electricallyconnects the main winding 214 to the electrical load 103. The taps 215are separated from each other on the main winding 214, and the outputvoltage of the voltage regulator 210 depends on the location of theselected tap on the main winding 214. Thus, by controlling which of thetaps 215 is connected to the contact or contacts that carry the loadcurrent, the output voltage to the load 103 is also controlled. In thisway, the voltage delivered to the electrical load 103 may be kept withinthe acceptable or desired range even if the voltage delivered from thepower source 102 changes.

The electromagnetic circuit 220 includes current paths 225. The currentpaths 225 are any electrically conductive path that is able to conductcurrent from the contacts 224 to the load 103. The current paths 225 maybe any type of electrical cable, transmission line, or wire. Theelectromagnetic circuit 220 also includes a winding 221, which iswrapped around a magnetic core 223 and is also electrically connected toone of the contacts 224. The magnetic core 223 is similar to themagnetic core 123 (FIGS. 1A-1C), and may be an un-gapped or gappedmagnetic core. The electromagnetic circuit 220 also includes a secondarywinding 251. The secondary winding 251 is also wrapped around themagnetic core 223. Thus, the winding 221 and the secondary winding 251are magnetically coupled, and, a time-varying current that flows in thesecondary winding 251 induces a corresponding time-varying current inthe winding 221.

The electromagnetic circuit 220 also includes a current controlapparatus 250. The current control apparatus 250 is electricallyconnected to the secondary winding 251. The current control apparatus250 controls the characteristics of a time-varying current that flows inthe secondary winding 251, thereby controlling an induced current in thewinding 221 and thus also controlling the current in the contact 224electrically connected to the winding 221. Furthermore, by controllingthe current in the secondary winding 251 during a switching operation,saturation of the core 223 may be avoided. The voltage regulator 210includes an on-load tap changer, meaning that the loads 103 remainconnected to the source 102 when an electrical contact 224 is removedfrom one of the taps 215 and when the electrical contact 224 isconnected to one of the taps 215. Because the loads 103 remainconnected, removing and/or connecting an electrical contact 224 maygenerate an arc, which reduces the lifetime of the electrical contact224. The current control apparatus 250 controls the current in theelectrical contact 224. By controlling the current in the electricalcontact 224, the current control apparatus 250 results in reduced oreliminated arcing and a longer lifetime for the voltage regulator 210.Additionally, the current control apparatus 250 is electrically isolatedfrom the main winding 214. The electrical isolation allows low-voltagedevices (for example, transistors) to be used in the current controlapparatus 250, thus reducing costs and complexity.

The voltage regulator 210 also includes a sensor 265 that measuresvoltage and current in various portions of the electromagnetic circuit220 and/or to the electrical load 103. The sensor 265 may be locatedanywhere along the current paths 225. In some implementations, theelectromagnetic circuit 220 includes more than one sensor 265. Thesensor 265 provides data to a controller 260 via a data link 259. Thedata link 259 may be any path capable of transmitting data. For example,the data link 259 may be a network cable (such as an Ethernet cable), orthe data link 259 may be a wireless connection that is capable oftransmitting data.

The controller 260 may be implemented as an electronic controller thatincludes one or more electronic processors and an electronic memorycoupled to the electronic processor. The controller 260 also may includemanual or electronic user interface devices that allow an operator ofthe voltage regulator 210 to communicate with the controller 260. Thecontroller 260 may store instructions, perhaps in the form of a computerprogram, on the electronic storage. The instructions may relate tomanipulation of data received from the sensor 265. For example, theinstructions may include a program or process that analyzes voltageand/or current data over a period of time to determine a time-rate ofchange of voltage and/or current. The electronic storage may storethreshold data and instructions to compare determined rates of changewith thresholds. The controller 260 also may interact with the currentcontrol apparatus 250. For example, the controller 260 may producesignals that, when received by the current control apparatus 250, aresufficient to cause electronic components within the apparatus 250 toperform certain actions.

FIGS. 3A-3E are block diagrams of an example of a load tap changer 301.Each of the FIGS. 3A-3E shows the load tap changer 301 at a differenttime. The load tap changer 301 is part of a voltage regulator 310. FIG.3A shows an example of the load tap changer 301 operating insteady-state. FIG. 3B shows the load tap changer 301 at a time justprior to breaking a connection to a tap. FIG. 3C shows the load tapchanger 301 during a load tap change operation (or a switchingoperation) and while the primary contact is not connected to a tap. FIG.3D shows another example of the load tap changer 301 operating insteady-state. FIG. 3E shows another example of the load tap changer 301during a switching operation.

The voltage regulator 310 may be used in the power distribution network101 (FIG. 1A) to deliver power from the source 102 to the loads 103. Thevoltage regulator 310 includes an electromagnetic circuit 320 thatdelivers a load current 381 to the electrical load 103, and a currentcontrol apparatus 350 that controls the current in the electromagneticcircuit 320 and the magnetic flux in the core 323. Controlling thecurrent in the electromagnetic circuit 320 enables more efficientoperation of the voltage regulator 310 and extends the lifetime of theload tap changer 301. For example, in some implementations (includingthe implementation discussed in FIGS. 3A-3E), the current controlapparatus 350 enables a contact to be separated from a tap with no orminimal arcing. Reducing the amount of arcing increases the lifetime ofthe contacts. Additionally, in some implementations, controlling themagnetic flux results in no or minimal in-rush currents when a contactis connected to a tap. This also increases the lifetime of the contacts.

The voltage regulator 310 includes a shunt winding 312 and a mainwinding 314 (or series winding 314). The shunt winding 312 is inparallel with the source 102, and the main winding 314 is in series withthe load 103. The main winding 314 includes at least two taps (taps 315a and 315 b are shown in the example of FIGS. 3A-3E). The main winding314 also includes a neutral point 317. Connecting a contact 324 a or 324b to the neutral point 317 causes the load 103 to be energized at thevoltage provided by the source 102 without voltage addition orsubtraction from the main winding 314. A first end 318 or second end 319of the main winding 314 is electrically connected to the source 102. Asin contemporary voltage regulators, the locations of the source 102 andthe load 103 may be reversed, but the voltage regulating function issimilar.

The voltage regulator 310 also includes an electromagnetic circuit 320that is electrically connected to the electrical load 103 via a node380. The electromagnetic circuit 320 includes a first electricalconductor 321 a that is electrically connected to a first contact 324 aand to the node 380. The electromagnetic circuit 320 also includes asecond electrical conductor 321 b that is electrically connected to asecond contact 324 b and to the node 380. The first electrical conductor321 a, the second electrical conductor 321 b, the first contact 324 a,and the second contact 324 b are made of electrically conductivematerial. For example, the first electrical conductor 321 a may be ametal wire or cable, and the first contact 324 a may be formed at an endof the wire or cable. When either of the first contact 324 a or thesecond contact 324 b is connected to one of the taps 315 a, 315 b, theelectromagnetic circuit 320 electrically connects the main winding 314to the node 380 and delivers a load current 381 to the electrical load103.

The voltage regulator 301 includes the taps 315 a, 315 b and thecontacts 324 a, 324 b. The output voltage of the voltage regulator 301is the voltage of the source 102 plus the voltage between the selectedtap and the neutral point 317. Thus, the output voltage of the mainwinding 314 is determined by which tap 315 a, 315 b is connected to thecontact that carries the load current 381. Both of the contacts 324 a,324 b may be movable contacts that are capable of contacting either ofthe taps 315 a, 315 b. However, in the example discussed below, thecontact 324 b is the primary contact that generally carries the loadcurrent 381, and the contact 324 b is moved between the taps 315 a, 315b.

The electromagnetic circuit 320 also includes the current controlapparatus 350. The current control apparatus 350 includes a secondarywinding 351 that is wrapped around a magnetic core 323, and anelectrical network 352 that is configured to control the voltage acrossthe secondary winding 351 and the current through the secondary winding351. The current through the secondary winding 351 is referred to as thebias current 382. By controlling the current through the secondarywinding 351, the electrical network 352 allows control of the magneticflux in the magnetic core 323. For example, the electrical network 352is able to substantially prevent saturation of the magnetic core 323during switching of the first contact 324 a or the second contact 324 b,as discussed below. The electrical network 352 may include any type ofcurrent source that is able to produce a time-varying current having aparticular amplitude and phase. The electrical network 352 is controlledby the controller 260, which receives data that indicates an amplitudeand phase of the current that flows in the second electrical conductor321 b from the sensor 265 via the data link 259.

The first electrical conductor 321 a includes a winding 322 that is alsowrapped around the magnetic core 323. Thus, the secondary winding 351and the first electrical conductor 321 a are magnetically coupled, andwhen the bias current 382 flows in the secondary winding 351, acorresponding AC current is induced in the first electrical conductor321 a.

FIGS. 3A-3E show an example of a tap change operation performed tochange the output voltage of the voltage regulator 310 by selectingdifferent taps on the main winding 314. In the example discussed below,the contact 324 b is removed from the tap 315 b, moved toward the tap315 a, and placed in contact with the tap 315 a.

FIG. 3A, shows an example of steady-state operation of the voltageregulator 310. Steady-state operation is a normal operating condition inwhich the contacts 324 a, 324 b are stationary and neither of thecontacts 324 a, 324 b is in the process of being moved to another tap.During steady-state operation of the voltage regulator 310, theelectrical network 352 is controlled such that no current flows in thesecondary winding 351 and all of the load current 381 flows through thecontact 324 b. For example, the controller 260 may open a switch 366 tocreate an open circuit in the secondary winding 351. Alternatively, thefunction of the switch 366 may be realized in the electrical network352. In either case, no current flows in the secondary winding 351, andthe winding 322 and the magnetic core 323 can be designed so that thepath through the first electrical conductor 321 a is a high impedancepath. In contrast, the impedance of the second electrical conductor 321b is essentially zero (0), so the load current 381 flows through thecontacts 324 b and 321 b.

FIG. 3B shows the voltage regulator 310 at a time just prior toseparating the contact 324 b from the tap 315 b. An arc will form if theload current 381 is flowing through the contact 324 b when the contact324 b is separated from the tap 315 b. However, the current controlapparatus 350 prevents or mitigates arc formation. The controller 260receives a signal indicating that the output voltage of the voltageregulator 310 is to be changed. The controller 260 also receives datathat includes a measurement of the amplitude and phase of the currentthat flows in the second electrical conductor 321 b (the load current381) from the sensor 265. In preparation for separating the contact 324b from the tap 315 b, the controller 260 closes the switch 366 such thatthere is no longer an open circuit in the secondary winding 351, and thecontroller 260 causes the electrical network 352 to generate a biascurrent 382. The controller 260 controls the electrical network 352 suchthat the bias current 382 flows through winding 351 inducing acirculating current 383 through winding 322 having the same amplitudeand phase as the load current 381. The ratio of bias current 382 to thecirculating current 383 depends on the number of turns in the winding322 and the secondary winding 351. This relationship is well understoodby those who practice the art. The circulating current 383 adds to thecurrent 381 flowing in the second electrical conductor 321 b such thatwhen the amplitude and phase of the circulating current 383 is properlycontrolled, the sum of current in the contact 324 b is zero (0). Thecontact 324 b is then separated from the tap 315 b. Because no currentis flowing in the contact 324 b at the time of separation, an arc is notformed. In some implementations, the circulating current 383 is notprecise enough to cause the current in the contact 324 b to be preciselyzero (0). However, in these implementations, the presence of thecirculating current 383 reduces the current in the contact 324 b suchthat the root-mean-square (RMS) current in the contact 324 b is lessthan the load current 381 and some performance improvement may still berealized.

FIG. 3C shows the voltage regulator 310 after the contact 324 b hasseparated from the tap 315 b but before the contact 324 b has joined tothe tap 315 a. No current flows in the contact 324 b when the contact324 b is not connected to the tap 315 a or the tap 315 b. The shortcircuit path for current for the circulating current 383 has beenremoved. As a result, the contact 324 a must carry the entire loadcurrent 381 during the period in which the contact 324 b does not touchone of the taps 315 a, 315 b. To create a low impedance path through thewinding 322, the flux in the core 323 is controlled to zero (0) by theelectrical network 352. The flux in the core 323 is controlled to zeroby controlling the amplitude and phase of the bias current 382.Alternatively, the electrical network 352 is shorted such that itappears as a low impedance and has a negligible effect on the impedanceof winding 322.

FIG. 3D shows the voltage regulator 310 after the contact 324 b isconnected to the tap 315 a, and the load tap changer 301 returns tosteady-state operation. In the example shown in FIG. 3D, after thecontact 324 b is connected to the tap 315 a, the controller 260 opensthe switch 366 to create an open circuit in the secondary winding 351.The open circuit causes the impedance of the secondary winding 351 andthe first electrical conductor 321 a to be higher than the impedance ofthe second electrical conductor 321 b. Thus, all the load current beginsto flow through the contact 324 b to the node 380 again.

A procedure similar to that discussed above is used to separate thecontact 324 b from the tap 315 a. To move the contact 324 b back to thetap 315 b, the controller 260 closes the switch 366, and causes theelectrical network 352 to generate the bias current 382, which induces acirculating current 383 that has the same amplitude and phase as theload current 381 that flows in the second electrical conductor 321 b.The bias current 382 induces the circulating current 383 in the firstelectrical conductor 321 a, and the circulating current 383 cancels thecurrent that flows in the second electrical conductor 321 b. Thus,current stops flowing through the contact 324 b and the contact 324 bmay be removed from the tap 315 a without producing an arc.

Referring also to FIG. 3E, while the contact 324 b is not contacting thetap 315 a or the tap 315 b, the electrical network 352 is controlled toadjust the flux in the core 323. In particular, the electrical network352 adjusts the magnetic flux in the magnetic core 323 to substantiallyprevent saturation of the core 323. Saturation occurs when the magneticcore 323 reaches its flux carrying limit. When saturated, the magneticcore 323 is unable to carry more flux, and additional flux must becarried by the medium that surrounds the magnetic core 323 (free spacein this example). The medium that surrounds the magnetic core 323 has amuch lower magnetic permeability than the magnetic core 323. Thus, theeffective impedance (for example, the inductance) of the winding 322 isdramatically reduced. If the saturation condition is present when thecontact 324 b is connected to the tap 315 b (or makes with the tap 315b), the relatively low effective impedance caused by the saturation mayresult in generation of a circulating current that has a much greateramplitude than a typical circulating current, and the large circulatingcurrent may cause damage to the contacts 324 a, 324 b and/or othercomponents of the voltage regulator 310. As such, it is desirable tocontrol the magnetic flux in the magnetic core 323 during a switchingoperation to avoid saturation or lessen the likelihood of saturationoccurring.

The cause of saturation of the magnetic core 323 during a switchingoperation (in this example, while the contact 324 b is not on either thetaps 315 a, 315 b) is the state of the magnetic flux within the magneticcore 323 just prior to and immediately after the contact 324 b isconnected to the tap 315 b. For example, while the contact 324 b istransitioning to the tap 315 b (as shown in FIG. 3E), the contact 324 acarries all of the load current 381. The impedance of the winding 322 isin series with the impedance of the load 103, and a voltage drop (Vi)forms across the winding 322. The current flowing in the winding 322forms a time-varying magnetic field proportional to (Vi) in the magneticcore 323. When the contact 324 b makes with the tap 315 b, the voltagedrop across the winding 322 immediately becomes equal to a voltage Vt,which is the voltage difference between the tap 315 a and the tap 315 b.Because the winding 322 has an impedance that is essentially completelyinductive, and the load has an impedance that is mostly resistive withsome inductive components, the phase of Vi and Vt is generally not thesame. Thus, the flux in the magnetic core 323 may be within saturationlimits when the voltage Vi is the voltage across the winding 322, butthe additional change in flux imposed by the change in voltage to thevoltage Vt may cause the core 323 to saturate.

To prevent saturation, the electrical network 352 controls the flux inthe magnetic core 323 during the switching operation. For example, theelectrical network 352 controls the magnitude and phase of the currentthrough the winding (the bias current 382) to ensure that the flux inthe core 323 remains within the saturation limit when the contact 324 bis connected to the tap 315 b. In other words, prior to the contact 324b making with the tap 315 b, the electrical network 352 controls themagnitude and phase of the current 382 such that the flux in the core323 is adjusted to a phase and magnitude that will prevent saturationwhen the contact 324 b makes, thus causing the voltage of the windingbetween the tap 315 a and the tap 315 b to appear across winding 322.The electrical network 352 may control the flux in the core 323, by, forexample, reducing the absolute value of the magnetic flux in the core323. The absolute value of the magnetic flux in the core 323 may bereduced by allowing a current to flow in the winding 322. The magneticflux in the core 323 may be reduced by increasing or decreasing the biascurrent 382. In some implementations, the electrical network 352controls the flux in the magnetic core 323 to match an ideal fluxprofile that is to be achieved after the switching operation iscompleted.

Referring to FIG. 4, a block diagram of a voltage regulator 410 isshown. The voltage regulator 410 is another example of a voltageregulating electrical apparatus 110 that may be used with the electricalpower distribution network 101 (FIG. 1A). The voltage regulator 410includes the shunt winding 312, the main winding 314, and the load tapchanger 301, which includes the taps 315 a, 315 b, a first contact 324a, and a second contact 324 b. The voltage regulator 410 also includesan electromagnetic circuit 420 that electrically connects one or both ofthe taps 315 a, 315 b to the electrical load 103 to deliver a loadcurrent 481 to the electrical load 103. A current control apparatus 450that controls the current in the electromagnetic circuit 420 such thatthe current flowing through a contact connected to one of the taps 315a, 315 b is driven to zero prior to separating that contact from thetap.

The electromagnetic circuit 420 includes a first electrical conductor421 a, which includes a first winding 422 a that is wound around amagnetic core 423. The electromagnetic circuit 420 also includes asecond electrical conductor 421 b, which includes a winding 422 b thatis also wound around the magnetic core 423. Thus, the first electricalconductor 421 a and the second electrical conductor 421 b aremagnetically coupled and a time-varying current in the first electricalconductor 421 a induces a corresponding time-varying current in thesecond conductor 421 b, and vice versa. The first contact 424 a iselectrically connected to the first electrical conductor 421 a, and thesecond contact 424 b is electrically connected to the second electricalconductor 421 b. The electrical contacts 424 a, 424 b share the loadcurrent during steady state operation providing benefits over theimplementation shown in FIGS. 3A-3E, which will be apparent to thoseskilled in the art.

Under steady-state conditions, equal load current flows in the first andsecond electrical conductors 421 a, 421 b. A current 483 a flows in thefirst electrical conductor 421 a, and a current 483 b flows in thesecond electrical conductor 421 b. Because the first and secondelectrical windings 422 a, 422 b are magnetically coupled, the loadcurrent 481 divides evenly between the conductors 421 a, 421 b when thewindings 422 a, 422 b have the same number of turns. When the contact424 a is connected to the tap 315 a and the contact 424 b is connectedto the tap 315 b, a circulating current (Ix) flows in theelectromagnetic circuit 420 in addition to the load current 481 becauseof the voltage from main winding 314 existing between taps 315 a, 315 b.The circulating current travels in opposite directions in each of theelectrical conductors 421 a, 421 b. In the example of FIG. 4, thecurrent 483 a has an amplitude of half of the load current 481 plus thecirculating current (Ix), and the current 483 b has an amplitude of halfof the load current 481 minus the circulating current (Ix). Thus, thecirculating current balances out and is not delivered to the electricalload 103.

Historically, the magnetic core 423 was designed as a gapped core. Agapped core includes gaps of non-magnetic material between segments ofmagnetic material. The configuration of the gaps controls the impedanceof the windings 422 a and 422 b and determines saturationcharacteristics of the core 423. Generally, the windings 422 a and 422 bhave a relatively low impedance when a gapped core is used. Theconfiguration of the gaps is typically selected to produce a trade-offbetween circulating current and saturation of the core during switching.

On the other hand, the configuration and presence of the current controlcircuit 450 makes it possible to use an un-gapped core or a core with agap that is smaller than a typical gapped core. The core used for ahigh-current voltage regulator may have a total core gap of about one(1) inch to achieve the desired impedance. Using the current controlcircuit 450 may allow the reduction of the core gap to perhaps 1/10thinch or 1/100th inch or less. The result would be circulating currentreduction of approximately 90% or 99% or more, respectively. Thereduction in circulating current results in lower FR losses and thesmaller gap size may additionally reduce stray losses. This may lead toa reduction in losses of tens to hundreds of watts depending on the sizeof the voltage regulator. The current control apparatus 450 allows thecontrol of magnetic flux during switching to prevent saturation, asdiscussed above. As a result, the magnetic core 423 may be designedwithout gaps, thus allowing the windings 422 a and 422 b to have a highimpedance and to thereby effectively minimize the steady-statecirculating current substantially close to zero. The reduced circulatingcurrent results in less total current flowing in the contacts 424 a and424 b, thereby allowing the contacts 424 a,b to be designed for lowercurrent than previously required. Moreover, the high impedance of thewindings 422 a and 422 b reduces the electrical losses of theelectromagnetic circuit 420 as compared to a design that uses a gappedmagnetic core 423.

The electromagnetic circuit 420 also includes the current controlapparatus 450, which controls the current in the first conductor 421 aand/or the second conductor 421 b. The current control apparatus 450includes a secondary winding 451, which is wound around the magneticcore 423, and an electrical network 452. The electrical network 452 mayinclude an AC current source. Because the secondary winding 451 is woundaround the same magnetic core as the first and second windings 422 a,422 b, the secondary winding 451 is also coupled to the first and secondwindings 422 a, 422 b. Thus, a current that flows in the secondarywinding 451 induces a corresponding circulating current in the first andsecond electrical conductors 421 a, 421 b with characteristic similar tothe circulating current (Ix).

The electrical network 452 is coupled to the controller 260, whichreceives data that indicates the phase and amplitude of the current thatflows in the first electrical conductor 421 a and the second electricalconductor 421 b. During steady-state operation, the current controlapparatus 450 is not used to influence the current in theelectromagnetic circuit 420, and the secondary winding 451 may be opencircuited (for example, by opening a switch such as the switch 366 ofFIG. 3A). Just prior to removing a contact from a tap, the controller260 closes the switch such that current is able to flow in the secondarywinding 451.

An example of the operation of the current control apparatus 450 duringa tap change operation in which the contact 424 b is separated from thetap 315 b is discussed. The controller 260 receives an indication of anupcoming tap change operation and the controller 260 causes the switchto close so that a bias current 482 from the electrical network 452flows in the secondary winding 451. The bias current 482 is controlledto produce a circulating current (Ix) with a magnitude that is the sameas the magnitude as the current 483 b, and a phase that is opposite tothe phase of the current 483 b. The bias current 482 induces acorresponding current in the second electrical conductor 421 b. Thecorresponding current cancels the current that flows through the contact424 b such that no current flows in the contact 424 b, and the contact424 b is removed from the tap 315 b without generating an arc.

Referring also to FIG. 5, a plot 500 shows a simulated example of thecirculating current (Ix) produced by the bias current 482 relative tothe load current 481 and to half of the load current (labeled as 484).The curves labeled as Ix, 481, 484, 485 on the plot 500 representinstantaneous current amplitude (y axis) and phase (x axis) as afunction time. In the example of FIG. 5, prior to separation, thecontact 424 b carries a current that is labeled as 484. The current 484has an amplitude that is half of the amplitude of the load current 481and has a phase that is the same as the phase of the load current 481.The bias current 482 induces the circulating current Ix in the windings422 a, 422 b. The induced circulating current Ix has an equal amplitudeas the load current flowing in the second electrical conductor 421 b andis 180° out of phase. As such, the induced circulating current Ix drivesthe current in the second electrical conductor 421 b to zero (0). Thus,the second contact 424 b can be removed from the tap 315 b withoutgenerating an arc. The net current that flows in the second electricalconductor 421 b after the bias current 482 flows in the secondary coil451 is represented by the data labeled 485 in the plot 500.

Referring also to FIG. 6, a plot 600 shows a simulated example based onan implementation in which a low-amplitude, high-frequency current issuperimposed on the bias current 482 to form a bias current 482′. Thecurrent (Ix′) induced in the second electrical conductor 421 b also hasthe low-amplitude, high-frequency ripple. The load current 481 is notaffected by the presence of the low-amplitude, high-frequency ripple.Thus, the net current that flows in the second electrical conductor 421b (labeled as switched current 485′) also has the low-amplitude,high-frequency ripple. In the situation in which the bias current 482 isnot accurate enough to create a circulating current to completely cancelthe current 483 b, the ripple produces more frequent opportunities forzero crossings in the net current 485. A zero crossing in the netcurrent 485 represents an instance in time when there is no currentflowing in the second contact 424 b. A zero crossing is required toextinguish the arc of a traditional load tap changer. Thus, the presenceof the ripple reduces the duration of the arc and therefor reduces thearc energy as compared to a situation in which there is no ripple andthe bias current is not accurate enough to bring the net current in thesecond electrical contact 421 b to zero.

The low-amplitude, high-frequency current used to form the bias current482′ and the current Ix′ may have a frequency that is, for example, fourto twenty times the system frequency. For example, in an implementationin which the system frequency is 50 Hz, the high-frequency current mayhave a frequency of 200 Hz to 1000 Hz. In an implementation in which thesystem frequency is 60 Hz, the high-frequency current may have afrequency of 240 Hz to 1200 Hz. The amplitude of the low-amplitude,high-frequency current used to form the bias current 482′ may result ina switched current 485′, for example, of 5 to 20 Amperes (A).

Referring to FIG. 7A, a block diagram of another example voltageregulator 710 that includes the load tap changer 301 is shown. Thevoltage regulator 710 is an example of an implementation of the voltageregulator 410. The voltage regulator 710 includes an electrical network752. The electrical network 752 functions as an AC current source thatproduces the bias current 482. The voltage regulator 710 also includes acurrent control apparatus 750, which controls an amount of magnetic fluxin a magnetic core 423.

In the voltage regulator 710, the first electrical conductor 421 a andthe second electrical conductor 421 b are electrically connected to anequalizer winding 780 that is magnetically coupled to the main winding314. The equalizer winding 780 is also electrically connected to a node779 and the electrical load 103. Additionally, in the voltage regulator710, the bias current 482 is generated by the electrical network 752.The electrical network 752 is electrically connected to a winding 753,which is magnetically coupled to the shunt winding via a core 790 anddraws power from the shunt winding 312. Thus, the time-varying (AC)current in the shunt winding 312 from the source 102 induces acorresponding time-varying (AC) current 788 in the winding 753.Together, the winding 753, the winding 312, and their common core (thecore 790) form a voltage transformer.

The electrical network 752 includes a rectifier 754, which converts theAC current 788 that flows in the winding 753 to a direct current (DC), aDC link 755 (or DC bus 755), and an inverter 756, which converts DCenergy stored in the DC link 755 into AC current to produce the biascurrent 482. The DC link 755 stores DC energy and regulates a currentripple between the rectifier 754 and the inverter 756. The DC link 755may include one or more capacitors and/or inductors.

The rectifier 754 is any type of electrical network that is capable ofconverting an AC current into a DC current. The rectifier 754 mayutilize controlled switches such that it can return power from the DClink 755 to the AC power system 100 through the winding 753, which ismagnetically coupled to the shunt winding 312. The controlled switchesmay be, for example, transistors, such as, MOSFETS, BJTs, and/or IGBTs.Thus, in implementations in which controlled switches are used in therectifier 754, the rectifier 754 serves two purposes. First, therectifier converts AC current into DC current that is supplied to the DClink 755, which stores energy that the inverter 756 uses to produce thebias current 482. Second, the rectifier 754 is able to compensatereactive power from the power distribution network 101. In other words,the rectifier 754 is able to accept reactive power, which may beexpressed in units of volt-ampere reactive (VAr), and to providereactive power to the power distribution network 101. The ability of therectifier to compensate reactive power improves the power factor in thepower distribution network 101. Thus, the rectifier implemented withcontrollable switches allows a single apparatus (the rectifier) to servemore than one purpose, thereby reducing the need for additionalcomponents and providing a more efficient design.

The inverter 756 is any type of electrical network that converts the DCenergy in the DC link into an AC current (the bias current 482). In someimplementations, the rectifier 754 and the inverter 756 are implementedas two H-bridges. FIG. 7B is a block diagram of an example currentsource 752B. The current source 752B is an example implementation of acurrent source that may be used as the electrical network 752. Thecurrent source 752B includes a rectifier 754B and an inverter 756Bcoupled to each other by a DC link 755B. The rectifier 754B and theinverter 756B are implemented as H-bridges.

An H-bridge is a circuit that includes four (4) switches. The switchesmay be, for example, transistors, diodes, or any other mechanism thatmay be configured to allow current to flow or to prevent the flow ofcurrent. In the example of FIG. 7B, the rectifier 754B includes switchesSR_1, SR_2, SR_3, and SR_4. The inverter 756B includes switches SI_1,SI_2, SI_3, and SI_4. The DC link 755B is a capacitor that iselectrically connected between the rectifier 754B and the inverter 756B.

FIG. 7C is a block diagram of a current source 752C. The current source752C may be used as the electrical network 752 (FIG. 7A). The currentsource 752C generates the bias current 482 directly from the ACelectrical power that flows in the winding 753. As shown in FIG. 7A, thewinding 753 is magnetically coupled to the core 790 (FIG. 7A), whichdraws power from the shunt winding 312. The source 102 provides an ACcurrent that flows in the shunt winding 312. Thus, the AC current in thewinding 753 is current that is induced due to electrical power that isalready present in the core 790.

The current source 752C includes switches S1C, S2C. The switches S1C,S2C have at least two stable states, one state in which the switchconducts current and another state in which the switch does not conductcurrent. The switches S1C, S2C may be, for example, transistors, such asMOSFETS, BJTs, and/or IGBTs. The switches S1C, S2C may be controlled,for example, by controlling the voltage at the gate of the transistor.

The winding 753 is electrically connected to the secondary winding 451via an electrical conductor 727 c. The switch S1C is electricallyconnected to a first terminal 753 a of the winding 753 via an electricalconductor 727 a. The switch S2C is electrically connected to a secondterminal 753 b of the winding 753 via an electrical conductor 727 b.Both of the switches S1C, S2C are electrically connected to thesecondary winding 451 via an electrical conductor 727 d. Controlling thestate of the switches S1C, S2C determines the polarity of the voltageacross the secondary winding 451 and the direction of the bias current482. For example, the bias current 482 flows in a first direction whenthe switch S1C conducts current and the switch S2C does not conductcurrent, and the bias current 482 flows in the opposite direction whenthe switch S1C does not conduct current and the switch S2C conductscurrent.

The current source 752C uses the AC current that flows in the winding753 to generate the bias current 482 instead of using an inverter, suchas the inverter 756 (FIG. 7A). In other words, the winding 753 acts asan AC current source. Thus, in implementations of the voltage regulator710 (FIG. 7A) in which the current source 752C is used as the electricalnetwork 752, the electrical network 752 is directly powered by an ACcurrent source.

Referring to FIG. 7D, a block diagram of a voltage regulator 710D isshown. The voltage regulator 710D is another example of animplementation of the voltage regulator 410 (FIG. 4). The voltageregulator 710D is similar to the voltage regulator 710 (FIG. 7A), exceptthe voltage regulator 710D does not include the winding 753, and thevoltage regulator 710D includes a current control apparatus 750D insteadof the current control apparatus 750.

The current control apparatus 750D includes a current source 752D. Thecurrent source 752D is electrically connected to the equalizer winding780. The AC current source 752D includes switches S1D, S2D. The switchesS1D, S2D may be, for example, transistors. The switch S1_D iselectrically connected to an electrical conductor 727 a_D. Theelectrical conductor 727 a_D is connected to the conductor 421 b, whichis electrically connected to a terminal 780 a of the equalizer winding780. The switch S2D is electrically connected to an electrical conductor727 b_D, which is electrically connected to a terminal 780 b of theequalizer winding 780. The switches S1D, S2D are also electricallyconnected to the secondary coil 451. The equalizer winding 780 iselectrically connected to the secondary winding 451 via an electricalconductor 727 c_D. Controlling the state of the switches S1D, S2Ddetermines the polarity of the voltage across the secondary winding 451and the direction of the bias current 482. For example, the bias current482 flows in a first direction when the switch S1D conducts current andthe switch S2D does not conduct current, and the bias current 482 flowsin the opposite direction when the switch S1D does not conduct currentand the switch S2D conducts current.

The current source 752D uses the AC current that flows in the equalizerwinding 780 to generate the bias current 482 instead of using aninverter, such as the inverter 756 (FIG. 7A). In other words, theequalizer winding 780 acts as an AC current source, and the currentsource 752D is directly powered by an AC current source.

Referring to FIG. 8A, a block diagram of a voltage regulator 810 thatincludes the load tap changer 301 is shown. The voltage regulator 810may be used in the power distribution network 101 (FIG. 1A) to deliverpower from the source 102 to the electrical load 103. The load tapchanger 810 includes the electromagnetic circuit 420, which electricallyconnects the source 102 to the electrical load 103 by connecting thecontact 424 a and/or the contact 424 b to a tap. The voltage regulator810 also includes a current control apparatus 850, which controls anamount of magnetic flux in a magnetic core 423.

The current control apparatus 850 includes an electrical network 857, asecondary winding 851 that is electrically connected to the electricalnetwork 857, and a sensor 265 that is configured to measure the voltageacross the secondary winding 851 and/or the current in the secondarywinding 851 and/or flux in the core 423. The electrical network 857includes one or more electronic components configured to short thesecondary winding 851. For example, the electrical network may include acontrollable electronic switch, such as a transistor. Like the windings422 a, 422 b, the secondary winding 851 is wrapped around the magneticcore 423. Thus, the secondary winding 851 is magnetically coupled to thefirst winding 422 a and the second winding 422 b and to theelectromagnetic circuit 420. The sensor 265 is coupled to the controller260 via a data link 259. The sensor 265 is configured to providemeasurements of the current and/or voltage and or flux to the controller260. The controller 260 processes the measurements, and provides commandsignals to the current control apparatus 850.

The current control apparatus 850 eliminates or greatly reduces lossesrelated to a gapped magnetic core. Moreover, the current controlapparatus 850 makes it feasible to use an un-gapped magnetic core or amagnetic core that has a smaller than typical gap as the magnetic core423. Using a gapped magnetic core or a magnetic core with a smaller thantypical gap as the magnetic core 423 results in a higher impedance forthe windings 422 a and 422 b, leading to lower losses and lesscirculating current in steady-state. Un-gapped magnetic cores and coresthat have a smaller than typical gap are generally more prone tosaturation during switching. However, by controlling the magnetic fluxin the core 423, the current control apparatus 850 is also able toprevent saturation of the core 423 during a switching operation inimplementations in which an un-gapped magnetic core or a magnetic corewith a smaller than typical gap is used as the magnetic core 423.

Under steady-state conditions, both of the contacts 424 a, 424 b areconnected to the same tap or adjacent taps, the electrical network 857is an open circuit, and current does not flow in the secondary winding851. The contact 424 b is separated from the tap 315 a and moved to thetap 315 b. In this position, the output voltage at node 480 is theaverage of the taps 315 a, 315 b if windings 422 a, 422 b have the samenumber of turns. Subsequently, the contact 424 a may be separated fromthe tap 315 a and moved to the tap 315 b so that both of the contacts424 a, 424 b make contact with the tap 315 b. Only the movement of thecontact 424 b is discussed in the example below.

When the contact 424 b is separated from the tap 315 a, an arc is formedbecause, unlike the current control apparatuses 350, 450, and 750, thecurrent control apparatus 850 does not reduce the current in the contact424 a prior to separation. After the contact 424 b separates from thetap 315 a and the arc is interrupted, all load current is transferred tothe contact 424 a, and the voltage across the secondary winding 851changes (for example, increases) rapidly. The sensor 265 measures thevoltage across the secondary winding 851 over time, and provides themeasurement to the controller 260. The controller 260 determines thetime-rate-of-change of the voltage (dV/dt) based on at least two voltagemeasurements taken at different times and compares the dV/dt to athreshold. If the dV/dt exceeds the threshold, the controller 260 causesthe electrical network 857 to short the secondary winding 851.

For example, the electrical network 857 may include a transistor thatshorts the secondary winding 851 when in an ON state and forms an opencircuit when in an OFF state. In this example, the controller 260generates a trigger signal in response to determining that the dV/dtexceeds the threshold and provides the trigger signal to the gate of thetransistor. The trigger signal is sufficient to cause the transistor toturn ON, and the secondary winding 851 is shorted. Shorting thesecondary winding 851 provides a very low impedance path for electricalcurrent. Because the secondary winding 851 is magnetically coupled tothe magnetic core 423, the secondary winding 851 draws magnetic flux outof the magnetic core 423 and reduces the impedance of theelectromagnetic circuit 420 by conducting current in the secondarywinding 851 and the electrical network 857.

When the contact 424 b makes contact with the tap 315 b, the contact 424a is still connected to the tap 315 a, and the secondary winding 851 isstill shorted. A voltage difference between the tap 315 a and the tap315 b creates a circulating current in the electromagnetic circuit 420,and the circulating current induces a current in the secondary winding851. Thus, the current in the electromagnetic circuit 420 and thesecondary winding 851 changes (for example, increases) rapidly. Thesensor 265 measures the current in the secondary winding 851 over aperiod of time, and provides the current data to the controller 260. Thecontroller 260 determines the time-rate-of-change of the current(di/dt), and compares the di/dt to a threshold. A di/dt that exceeds thethreshold is an indication that the contact 424 a has connected to thetap 315 b and that the secondary winding 851 should no longer beshorted. If the di/dt exceeds the threshold, the controller 260 providesa trigger signal to the electrical network 857 that is sufficient toform an open circuit in the electrical network such that no currentflows in the secondary winding 851. Continuing with the example of theelectrical network 857 including a transistor, the trigger signal is asignal that is provided to the gate of the transistor and is sufficientto cause the transistor to switch from the ON state to the OFF state.After the transistor is turned OFF, the impedance of the windings 422 a,422 b increases, and the electromagnetic circuit 420 returns tosteady-state operation.

Although the current control apparatus 850 does not reduce the currentthat flows in the contact 424 b to zero (0) prior to removing thecontact 424 b from the tap 315 a, the current control apparatus 850still increases the lifetime of the contact 424 b as compared to aconventional load tap changer that lacks the current control apparatus850. For example, by shorting the secondary winding 851 during aswitching operation when only one of the contacts 424 a, 424 b isconnected to a tap, the current control apparatus 850 reduces themagnetic flux in the magnetic core 423 and reduces the impedance of theelectromagnetic circuit 420 during the switching operation. Thereduction in magnetic flux reduces the likelihood of the core 423saturating when the contact 424 b is connected to the tap 315 b andthereby reduces or prevents inrush currents (or surge currents) thatwould otherwise occur when the contact 424 b is connected to the tap 315b. By reducing or preventing inrush currents, the current controlapparatus 850 prolongs the lifetime of the contact 424 b and the voltageregulator 810. Further, the current control apparatus allows for the useof a high impedance electromagnetic circuit to minimize circulatingcurrent, which substantially reduces the amount of current the contactsmust interrupt, especially at lower load current levels, such that thecontact erosion from arcing is reduced. Moreover, the low circulatingcurrent contributes to a less inductive power factor to generallyimprove arc interruption. Less inductive power factor combined with theshorting of winding 851, which will reduce recovery voltage afterarcing, improves the arc interrupting capability of a load tap changer.

FIG. 8B is a block diagram of an electrical network 857B. The electricalnetwork 857B is an example implementation of the electrical network 857.The electrical network 857B includes a transistor 891 and a rectifierbridge 892 made from diodes D1-D4. The transistor 891 may be any type oftransistor, for example, a metal-oxide semiconductor field-effecttransistor (MOSFET) or an insulated-gate bipolar transistor (IGBT). Thetransistor 891 receives a trigger at a gate 893, and the trigger issufficient to cause the transistor 891 to change state. The trigger maybe received from the controller 260. When the transistor 891 is ON,current flows through the transistor 891 and the diodes D1-D4 conductcurrent so the secondary winding 851 is shorted. The diodes D1-D4 workin pairs, with the first pair being diodes D1 and D3, and the secondpair being diodes D2 and D4. Current flow through one of the two pairsduring the positive half of the cycle and the other of the two pairsduring the negative half of the cycle. When the transistor is OFF, theelectrical network 857B is an open circuit, and no current flows throughthe winding 851.

FIG. 9A is a block diagram of another example of a voltage regulator 910that includes the load tap changer 301. The voltage regulator 910 may beused in the power distribution network 101 (FIG. 1A) to deliver powerfrom the source 102 to the electrical load 103. The voltage regulator910 includes the electromagnetic circuit 420, which electricallyconnects the source 102 to the node 480 and the electrical load 103 byconnecting the contact 424 a and/or the contact 424 b to a tap on themain winding 314. A current transformer 930 is between the node 480 andthe electrical load 103. The current transformer 930 includes a firstcurrent winding 932, which is electrically connected to the node 480 andthe electrical load 103, and a second current winding 933. The firstcurrent winding 932 and the second current winding 933 are wrappedaround a magnetic core 931. Thus, when the load current 481 flowsbetween the node 480 and the electrical load 103 and in the firstcurrent winding 932, a corresponding time-varying current is induced inthe second current winding 933.

The voltage regulator 910 also includes a current control apparatus 950,which controls the current in the electromagnetic circuit 420. Thecurrent control apparatus 950 includes an electrical network 996, aphase network 997, and a secondary coil 951. The secondary winding 951is wrapped around the magnetic core 423, and is thus magneticallycoupled to the windings 422 a, 422 b of the electromagnetic circuit 420.The electrical network 996 is electrically connected to the secondcurrent winding 933 and to the phase network 997. The phase network 997is electrically connected to the secondary winding 951.

The electrical network 996 includes a shorting circuit, which may beclosed (or gated on) or opened (gated off). When the shorting circuit isclosed, the electrical network 996 reduces the magnetic flux in themagnetic core 931 and prevents saturation of the magnetic core 931. Whenthe shorting circuit is open, the current that is induced in the secondcurrent winding 933 may flow through the phase network 997 to form thebias current 982. The phase network 997 is one or more electroniccomponents arranged to form an electrical network that determineswhether the bias current 982 is able to flow to the secondary winding951 and also controls the direction that the bias current 982 flows inthe secondary winding 951.

FIG. 9B shows a current control apparatus 950B, which is an exampleimplementation of the current control apparatus 950. The current controlapparatus 950B includes an electrical network 996B, which is implementedin the same manner as the electrical network 857B of FIG. 8B, and aphase network 997B, which is implemented as H-bridge formed fromswitches S1-S4. The switches S1-S4 are used to control the direction inwhich current flows through the secondary winding 951. When the switchesS1 and S4 are closed and the switches S2 and S3 are open, current flowsthrough the secondary winding 951 with a first phase convention, forinstance zero (0) degrees. When the switches S1 and S4 are open and theswitches S2 and S3 are closed, current flows through the secondarywinding 951 with the opposite phase convention, for instance 180degrees.

In steady-state operation, both of the contacts 424 a, 424 b makecontact with one of the taps 315 a, 315 b. The contact 424 a conductsthe current 483 a, and the contact 424 b conducts the current 483 b.Each of the currents 483 a, 483 b are half of the load current 481. Thephase-inverting network 997 is in a configuration that does not conductcurrent and the bias current 982 does not flow in the secondary winding951. For example, in implementations in which the phase-invertingnetwork 997 is implemented as shown in FIG. 9B, all of the switchesS1-S4 are open during steady-state operation. Additionally, the shortingcircuit of the electrical network 996 is gated on (for example, thetransistor 891 is ON) such that the current transformer 930 does notsaturate or create an impedance between the tap changer 910 and theelectrical load 103. The functionality of the electrical network 996Bmay also be realized within the phase network 997B given the properswitch topology. For instance, switching on only S1-S2 or S3-S4 willshort winding 933 and leave winding 951 open.

The current control apparatus 950 is able to drive the current in eitherthe contact 424 a or the contact 424 b to zero prior to a switchingoperation by producing the bias current 982 and controlling thedirection of the bias current 982. The bias current 982 is current thatis induced in the second current winding 933 and flows into thesecondary winding 951 via the phase network 997. The bias current 982induces a circulating current in windings 422 a, 422 b having anamplitude that is half of the amplitude of the load current 481. Thebias current 982 has the same phase as the load current 481 because thebias current 982 is a current that is induced by the load current 481.Proper coordination of switches in phase network 997 causes thecirculating current to cancel current through contacts 424 a or 424 b.In preparation for performing a tap change operation, the shortingcircuit in the electrical network 996 is opened (for example, thetransistor 891 is switched to an OFF state), and the phase-invertingnetwork 997 is configured to allow the bias current 982 to flow throughthe secondary winding 951. The bias current 982 induces a correspondingcurrent in the electromagnetic circuit 420. The corresponding currentcauses the current on the contact 424 b to drop to zero, and all of theload current flows in the contact 424 a. The contact 424 b is thenremoved from the tap 315 a. An arc is not formed because no currentflows through the contact 424 b immediately prior to separation.

After the contact 424 b has separated from the tap 315 a, a rapid changein voltage occurs in the electrical network 996, and the electricalnetwork 996 is closed (for example, the transistor 891 is switched to anON state) to prevent saturation of the magnetic cores 423 and 931. Whilecontact 424 b is transitioning from tap 315 a to 315 b, the electricalnetwork 996 and phase network 997 can be coordinated to control the fluxof the magnetic cores 423, 931 to zero (0) to avoid saturation.Alternatively, the electrical network 996 and phase network 997 can becoordinated to control the flux of the magnetic core 423 with anamplitude and phase to prevent saturation when contact 424 b makes ontap 315 b. Once the contact 424 b makes on tap 315 b, the electricalnetwork 996 and phase network 997 are returned to steady stateconditions.

Thus, the current control apparatus 950 mitigates arc formation when acontact separates from a tap. Additionally, the current controlapparatus 950 prevents or reduces the likelihood of core saturationduring switching, and thus also mitigates or prevents in-rush currentswhen a contact makes contact with a tap. Moreover, the current controlapparatus 950 generates the bias current 982 at the correct amplitudeand phase without using separate current-generation devices and withoutusing a DC link or bus.

FIG. 10A is a block diagram of another example of a voltage regulator1010 that includes the load tap changer 301. The voltage regulator 1010may be used in the power distribution network 101 (FIG. 1A) to deliverpower from the source 102 to the electrical load 103. The voltageregulator 1010 includes an electromagnetic circuit 1020, whichelectrically connects the source 102 to the node 1079 and the electricalload 103 by connecting the contact 424 a and/or the contact 424 b to atap on the main winding 314. The electromagnetic circuit 1020 is thesame as the electromagnetic circuit 420, except the electromagneticcircuit 1020 includes two magnetic cores 1023 a and 1023 b. The winding422 a is wrapped around the core 1023 a, and the winding 422 b iswrapped around the core 1023 b. Additionally, in the voltage regulator1010, the first electrical conductor 421 a and the second electricalconductor 421 b are electrically connected to an equalizer winding 1080that is magnetically coupled to the main winding 314 and electricallyconnected to the electrical load 103.

The voltage regulator 1010 also includes a current control apparatus1050 that is configured to magnetically couple to the electromagneticcircuit 1020 to control the current flow in the electromagnetic circuit1020. The current is controlled prior to removing a contact from a tapto mitigate or prevent arcing.

The current control apparatus 1050 includes a first secondary winding1051 a, which is wrapped around the core 1023 a, and a second secondarywinding 1051 b, which is wrapped around the core 1023 b. Thus, the firstsecondary winding 1051 a is magnetically coupled to the first winding422 a via the core 1023 a, and the second secondary winding 1051 b ismagnetically coupled to the second winding 422 b via the core 1023 b.The core 1023 a and the core 1023 b are un-gapped magnetic cores orcores that include a smaller than usual gap.

The current control apparatus 1050 also includes an electrical network1052 that is electrically connected to the first secondary winding 1051a and the second secondary winding 1051 b. The electrical network 1052is configured to control an amount of current that flows in and/orvoltages across the first winding 422 a and the second winding 422 b.

FIG. 10B includes an electrical network 1052B, which is an example of animplementation of the electrical network 1052. The electrical network1052B includes a switch 1037, which is in parallel with the firstsecondary winding 1051 a, a switch 1038, which is in parallel with thesecond secondary winding 1051 b, and a by-pass switch 1036, which iselectrically connected to the first secondary winding 1051 a and thesecond secondary winding 1051 b. The by-pass switch 1036 is electricallyconnected to the switches 1037 and 1038, and the by-pass switch 1036 ispositioned between the switches 1037 and 1038. The switches 1036, 1037,and 1038 may be any type of electronic component that may be controlledto permit or allow current flow. For example, the switches 1036, 1037,and 1038 may be transistors. The state of the switches 1036, 1037, 1038may be controlled by the controller 260.

During steady-state operation, the switches 1037 and 1038 are open (suchthat no current flows through these switches), and the switch 1036 isclosed (such that current flows through 1036). Both of the contacts 424a and 424 b are on the same tap (the tap 315 a in the example of FIG.10B), and the currents 483 a and 483 b flow through the contacts 424 aand 424 b, respectively. Because the cores 1023 a and 1023 b are notgapped cores, there is little to no circulating current in thesteady-state. Equal current flows through the contacts 424 a and 424 b.Thus, the currents 483 a and 483 b have an amplitude that is half of theamplitude of the load current 1081. Moreover, because there is nocirculating current, the electromagnetic circuit 1020 and the currentcontrol apparatus 1050 have lower losses than a conventional voltageregulator with load tap changer, and the magnitude of current flowing inthe contacts 424 a, 424 b and in the first and second electricalconductors 421 a, 421 b is smaller than in a conventional load tapchanger. As a result, the contacts and conductors may be smaller andotherwise designed for operation at a lower than typical current.

An operation that moves the contact 424 b from the tap 315 a to the tap315 b is discussed as an example. The operation begins by removing thecontact 424 b from the tap 315 a while the load current 1081 isdelivered to the load. Just prior to removing the contact 424 b from thetap 315 a, the controller 260 provides a trigger signal to the switch1036, a trigger signal to the switch 1037, and a trigger signal to theswitch 1038. The trigger signal to the switch 1037 causes the switch1037 to close. The trigger signals to the switches 1036 and 1038 causesthe switches 1036 and 1038 to open. For example, the switches 1036,1037, and 1038 may be transistors, and the trigger signals may betrigger signals provided to the gate of the transistor that aresufficient to cause the transistor to change state.

With the switches 1036, 1037, and 1038 configured in this manner, thefirst secondary winding 1051 a is shorted and provides a very lowimpedance path for the load current 1081 through winding 422 a. Becausethe first secondary winding 1051 a is magnetically coupled to themagnetic core 1023 a, the first secondary winding 1051 a draws magneticflux out of the magnetic core 1023 a by conducting current in the firstsecondary winding 1051 a. At the same time, second secondary winding1051 b is open circuited such that winding 422 b becomes a highimpedance path. All of the load current 1081 flows through the firstcontact 424 a, and the contact 424 b is removed from the tap 315 awithout forming an arc. While contact 424 b is transitioning from tap315 a to 315 b, switches 1036, 1037, 1038 are coordinated to control theflux in magnetic cores 1023 a, 1023 b to prevent saturation. After thecontact 424 b makes with tap 315 b, switches 1037, 1038 are opened andswitch 1036 is closed to complete the tap change process.

Other implementations of the electrical network 1052 are possible. Forexample, as shown in FIG. 10C, the electrical network 1052 may beimplemented as an electrical network 1052C that includes a first AC-ACconverter 1061_1 that couples the first secondary winding 1051 a to theshunt winding 312 and a second AC-AC converter 1061_2 that couples thesecond secondary winding 1051 b to the shunt winding 312. Anotherexample implementation of the electrical network 1052 is shown in FIG.10D. FIG. 10D includes an electrical network 1052D. The electricalnetwork 1052D includes an AC-AC converter 1061_3 that couples the firstsecondary winding 1051 a to the second secondary winding 1051 b.

The implementations discussed, for example, in FIGS. 3A-3E, 4, 7A, 8A,and 9A have a steady state condition with both contacts (for example,contacts 324 a, 324 b) connected to taps such that a fixed voltage ratioexists at the voltage regulator between the voltage from the sourceinput to the load output (for instance, the voltage across the shuntwinding 312 and the voltage at node 480 of FIG. 4) such that the steadystate performance is the same as conventional voltage regulatorperformance but with reduced losses and also with the aforementionedbenefit of tap changing. In the implementations of, for example, FIG. 4and FIG. 7A, the respective electromagnetic circuits 420, 720 cause adependent relationship between the load currents flowing through thecontacts 424 a and 424 b such that each contact 424 a, 424 b carries onehalf of the load current when windings 422 a and 422 b have an equalnumber of turns. Due to magnetic coupling, equal voltage exists acrosswinding 422 a and winding 422 b. The effect is that the output voltageat node 480 or 779 (respectively) is the average of the voltage atcontact 424 a and the voltage at 424 b.

An additional advantage is realized in the implementation of FIG. 10A.In particular, the implementation of FIG. 10A employs the currentcontrol apparatus 1050 to couple and decouple the windings 422 a and 422b. For example, as discussed with respect to FIG. 10B, manipulation ofswitches 1036, 1037 and 1038 influences the current and voltage of thetwo windings 1051 a, 1051 b. Manipulating switches 1036, 1037 and 1038at high frequencies and with controlled duty cycles influences theroot-mean-square voltage at node 1079. Whereas the implementations ofFIG. 4, FIG. 7A, and FIG. 9A have a fixed ratio between the input andoutput voltage while the contacts 424 a and 424 b are fixed, theimplementation of FIG. 10A allows for variability of the ratio based onthe control of the voltage across windings 1051 a and 1051 b. Whether bymanipulation of switches 1036, 1037, 1038 (or an alternativeconfiguration of the electrical network 1052), the input:output voltageratio of the voltage regulator 1010 can be manipulated without themoving contacts 424 a, 424 b. Additional filter components may be addedto the system to reduce harmonics in the output voltage.

The example in FIG. 10E shows another implementation of the currentcontrol apparatus 1050. In the implementation of FIG. 10E, the currentcontrol apparatus 1050 includes a winding 1053, a rectifier 1054, a DClink 1055, and an inverter 1056. The current control apparatus 1050 iselectrically connected to the winding 1053, which is magneticallycoupled to the shunt winding 312 via a core 1090 and draws power fromthe shunt winding 312. A time-varying (AC) current in the shunt winding312 from the source 102 induces a corresponding time-varying (AC)current 1088 in the winding 1053. The implementation of the currentcontrol apparatus 1050 shown in FIG. 10E, like the current controlapparatus 750 discussed with respect to FIG. 7A, is able to compensatereactive power from the power distribution network 101 in addition tohaving the functionality of the implementations of FIGS. 10A and 10B.

Other implementations are within the scope of the claims.

For example, FIG. 10A illustrates the electromagnetic circuit 1020 andthe current control apparatus 1050 being used in the voltage regulator1010. However, other applications and implementations are possible. Forexample, in the implementation shown in FIG. 11, the load tap changer301 is removed and the electromagnetic circuit 1020 is connecteddirectly to the main winding 314. The source 102 may be connected, forinstance, in the middle of the main winding 314, either winding end 318,319, or at any point along the main winding 314 depending on the desiredoutput range of the voltage regulator. In the configuration shown inFIG. 11, when the electromagnetic circuit is in a neutral state, thevoltage delivered to node 1079 may be the average of the voltage presentat the first end 318 and second end 319 of the winding 314 which may,for example, cause the output voltage to the load 103 be equal to theoutput voltage from the source 102. Manipulation of the current controlapparatus 1050 may, however, cause the output voltage to be increased ordecreased as the voltages of windings 422 a,b are controlled.

FIG. 12 is an example of applying the implementation of FIG. 11 to adistribution transformer with a primary winding 1212 and a secondarywinding 1214. The distribution transformer delivers a load current 1281to the load 103. Manipulation of the current control apparatus 1050changes the voltages of windings 422 a,b such that the effective turnsratio of the windings 1212, 1214 is adjusted and the voltage deliveredto the load 103 may be regulated. Windings 1212, 1214 in FIG. 12 share acommon neutral node as in a grounded-wye connection, but the principlealso applies to delta-connected or otherwise isolated windings.

The rectifier, inverter and DC bus components of the implementation inFIG. 10E may be similarly implemented with the topologies in FIG. 11 andFIG. 12 to provide reactive power compensation for the power system toimprove power factor. In addition to the single-phase power systemapplications, any of these implementations may be applied to athree-phase system. Within a three-phase application, the applicationmay benefit from economies of scale, especially for the switching orpower electronic components, where the number of switches for invertersand rectifiers per phase is reduced and the DC bus performance isthereby improved.

What is claimed is:
 1. A voltage regulator comprising: a main magneticcore; a main winding comprising a plurality of taps, the main windingconfigured to magnetically couple to the main magnetic core; a firstcontact configured to electrically connect to one of the plurality oftaps; a second contact configured to electrically connect to one of theplurality of taps; an auxiliary winding configured to magneticallycouple to the main magnetic core; and an electrical network electricallyconnected to the auxiliary winding, the electrical network comprisingone or more controllable power electronic components.
 2. The voltageregulator of claim 1, wherein the electrical network further comprises afirst power converter configured to compensate for reactive power. 3.The voltage regulator of claim 2, wherein the electrical network furthercomprises a direct current (DC) link electrically connected to the firstpower converter.
 4. The voltage regulator of claim 3, wherein theelectrical network further comprises a second power converterelectrically connected to the DC link and the first power converter, andwherein the DC link is between the first power converter and the secondpower converter.
 5. The voltage regulator of claim 4, wherein the firstpower converter comprises a rectifier, and the second power convertercomprises an inverter.
 6. The voltage regulator of claim 1, wherein theone or more controllable power electronic components comprises aplurality of transistors.
 7. The voltage regulator of claim 1, whereinthe electrical network is configured to receive reactive power from theauxiliary winding and to provide reactive power to the auxiliarywinding.
 8. The voltage regulator of claim 7, further comprising: asecond magnetic core; and a current control winding, wherein the firstcontact is electrically connected to a first winding, the second contactis electrically connected to a second winding, the first winding and thesecond winding are configured to magnetically couple to the secondmagnetic core, the current control winding is electrically connected tothe electrical network and is configured to magnetically couple to thesecond magnetic core, and the electrical network is further configuredto control current in one or more of the first electrical contact andthe second electrical contact.
 9. The voltage regulator of claim 8,wherein the electrical network is configured to prevent saturation ofthe second magnetic core.
 10. The voltage regulator of claim 9, whereinthe second magnetic core is a gapped magnetic core.
 11. The voltageregulator of claim 8, wherein the electrical network is configured tocontrol the current in the first contact such that the current in thefirst contact is zero during a switching operation.
 12. The voltageregulator of claim 11, wherein the electrical network comprises arectifier electrically connected to the auxiliary winding.
 13. Thevoltage regulator of claim 12, wherein the rectifier comprises aplurality of transistors.
 14. The voltage regulator of claim 13, furthercomprising: a direct current (DC) link electrically connected to therectifier; and an inverter electrically connected to the DC link and thecurrent control winding.
 15. A controller for a voltage regulator, thecontroller configured to: receive data from a sensor that measures anelectrical quantity in the voltage regulator; and control electricalcurrent in an electrical network of the voltage regulator based on thedata received from the sensor; wherein the electrical network ismagnetically coupled to a magnetic core of the voltage regulator, andcontrolling the electrical current in the electrical network controlsmagnetic saturation of the magnetic core.
 16. The controller of claim15, wherein the controller is configured to control electrical currentin the electrical network by controlling a state of at least oneelectronic switch in the electrical network.
 17. A method ofcompensating for reactive power using a voltage regulator, the methodcomprising: electrically coupling a main winding of a voltage regulatorto an alternating current (AC) electrical power distribution system;magnetically coupling the main winding to a first magnetic core;electrically connecting an electrical contact to a tap of the mainwinding; magnetically coupling an electrical network to a secondmagnetic core; and controlling the electrical network to compensate forreactive power from the electrical power distribution system.
 18. Themethod of claim 17, further comprising magnetically coupling a firstsecondary winding to the second magnetic core; and wherein the firstsecondary winding is electrically connected to the electrical contact;and controlling the electrical network further comprises controlling anelectrical current in the electrical contact.
 19. The method of claim18, further comprising controlling a state of a switch in the electricalnetwork; and wherein controlling the state of the switch controls theelectrical current in the electrical network to thereby control theelectrical current in the electrical contact.
 20. The method of claim17, wherein the second magnetic core is part of the first magnetic core.21. The method of claim 17, wherein controlling the electrical networkto compensate for reactive power comprises controlling one or more powerconverters in the electrical network.